Signal transmission system



July 4, 1939. A. c. NoRwlNE SIGNAL TRANSMISSION SYSTEM Filed April 28,1938 /LOSSER NE Twan/r WIE 3o l a WIW LOSSER/ /5 l m NETWURK /A/VENTORA. C. NORW/NE Patented July 4, 1939 UNITED STATES PATENT OFFICETelephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application April 28, 1938,Serial No. 204,763

Claims.

This invention relates to signal transmission systems and particularlyto compression and. expansion circuits for four-wire signal transmissionsystems.

5 One object of the invention is to provide a fourwire transmissionsystem with control circuits that shall operate not only for effectingcompression and expansion of the signals on the system but also forgoverning the over-all attenuation of the system to prevent singing.

Another object of the invention is to provide a four-wire transmissionsystem that shall control variolossers in the transmitting and receivingchannels to compress the transmitted signals and to expand the receivedsignals while governing the over-all attenuation of the system toprevent singing.

A further object of the invention is to provide a four-wire transmissionsystem that shall control variolossers in the transmitting and receivingchannels oppositely to expand received signals and compress transmittedsignals while gov- 'erning the overall attenuation of the system toprevent singing and that shall prevent simultaneous control of thevariolossers from the transmitting and receiving channels.

In many signal circuits, and especially in circuits connected to radiotransmitting and receiving terminals, it is very often desirable tocompress the signals received from a telephone line before transmissionto the radio apparatus in order to more completely load the spacedischarge devices in the transmitter and to expand the signals receivedfrom the radio apparatus in order to reduce the effective noise and toproduce undistorted signals. A compressor and expander arrangement ofthe type disclosed in the patent to S. Doba, Jr., 2,018,489, October 22,1935, or in the patent to L. Schott, 2,084,457, June 22, 1937, may beemployed in case part of the four-wire system comprises a radio circuit.

The transmission equivalent of four-wire systems on which compressor andexpander circuits are employed will vary by some amount due to changesin temperature and humidity and, in the case of a radio link, b-yfading. Changes in the transmission equivalent will give rise todiiculties because the expanders will magnify the variations caused inthe signals. Trouble is caused because the expander sets the gainaccordingto the received energy which must be the same as the energyleaving the compressor in order to obtain complementary gain settings.If the channel between a compressor and an expander changes so that ithas two decibels gain, then the expander obtains too great an inputwhich causes the expander to have two decibels too much gain if a two toone ratio expander is employed. If the channel Variations 'produce aloss, the expander will have too little gain and again accentuate theeffect of the variation.

Accentuation of the channel variations will cause that. part of thespeech energy which returns to the station of a talker as an echo to belouder or weaker than without a compressor expander arrangement. In theabsence of speech on a system equipped with compressor expanderequipment, noise within the system may cause one or both of theexpanders to increase gain. Echo currents around the whole system willbe 15 increased and, if the gain increase is suiiicient, sol-calledsinging will be produced.

In the above-mentioned patent to L. Schott, 2,084,457, singing on afour-wire system provided with a compressor expander equipment isprevented b-y means of an echo suppressor arrangement of relays.According to the present invention, singing is prevented by an interlockcircuit connection between the compressor and the expander at eachterminal station of the four-wire system.

The compressor at a terminal station is provided with a variolosser inthe form of an impedance pad having a copper-oxide bridge with twoopposite vertices connected across the transmitting channel at thestation. The loss produced by the variolosser is controlled by varyingthe current flow between the other two vertices of the bridge.

The expander at a terminal station is provided with a variolosser in theform of an impedance pad having copper-oxide rectier elements insertedin series with each side of the two-wire receiving channel. The losscaused by the variolosser is controlled by varying the current flowthrough the rectifier elements of the pad in the receiving channel.

An interlock circuit is` connected between the variolossers o-f thecompressor and the expander of each terminal station. Each interlockcircuit has two bridge circuits connected across it which are composedof copper-oxide elements or other suitable non-linear elements. Thebridge circuit adjacent to the variolosser in a compressor is governedby a backward-acting control circuit including an amplier and connectedto the transmitting channel beyond the variolosser of the compressor.The bridge circuit adjacent to the variolosser in an expander isgoverned by a forward-acting control circuit including an amplifierwhich is connected to the receiving channel before the variolosser ofthe expander. In the interlock circuit between the variolosserassociated with the compressor and the variolosser of the compressor isinserted a condenser resistance arrangement to produce a time constant.Another condenser resistance arrangement is inserted in the interlockcircuit between the bridge associated with the expander and thevariolosser of the expander to produce a time constant. The timeconstants of the condenser resistance arrangements are preferably of theorder of forty milliseconds.

'Ihe two bridges connected across the interlock circuit are poled sothat they produce the same polarity of control voltage across the leadsof the variolossers. Thus, control current fom either control circuitmay flow through each of the variolossers, changing their losses inopposite directions because their congurations .are such that the lossesare complementary. In a compressor expander arrangement, as aboveoutlined, noise or speech entering the expander input will cause itsgain to increase and will also cause the compressor gain to decrease. Aninput to the compressor will cause it to reduce the gain on thetransmitting channel and will also cause the expander to increase thegain on the receiving channel. When currents are received by theexpander, it may be noise currents or signal currents, the gain eiectedby the expander is compensated by an equal loss effected by thecompressor so that there can be no increased tendency for the system tosing. When the transmitting channel is in service, the speech currentsof the talker will compress transmitted signals but will also increasethe gain of the expander upon the receiving channel which thereby raisesthe noise which the talker may hear. However, it has been shown byexperience that this noise increasing feature, when the transmittingchannel is in service, is not objectionable even with fairly high noisecircuits if there is no appreciable delay in transmission because thenoise level is high only when the speech is high in amplitude. When thetalker is located at a distance from the compressor expander location,the noise variations are more readily perceptible.

In order to avoid raising the effective received noise when thesubscriber is talking, a modied unilateral interlock circuit may beprovided between the compressor and the expander at each terminalstation. In the modied interlock circuit between the variolossers of thecompressor and the expander, copper-oxide discs or similar one-waytransmitting means are provided in the interlock circuit between the twobridges so that the two variolossers may be controlled from thereceiving channel but not from the transmitting channel. In such aconnection, outgoing speech cannot vary the incoming noise but incomingspeech or noise will reduce the gain in the transmitting channel of thestation.

An additional variation in the proposed modiiication may be provided byinserting a biasing potential in series with the rectifier units in theinterlock circuit between the two bridges. The biasing potential is sopoled that the expander rectifier voltage must always exceed this biasin order to control the compressor variolosser. This additional biasingpotential may be advantageous if the system equipped with compressorexpander circuits were so noisy that the expander Control frequentlyreduced the compressor gain during outgoing weak speech currents.

The two bridges in the interlock circuit are so poled with respect toeach other as to prevent control of the two variolossers from both thetransmitting and receiving channels at the same time. If one bridgeproduces a greater voltage across the interlock circuit than does theother b-ridge, the second bridge is effectively biased and can supplylittle or no regulating current to the variolossers.

Fig. 1 in the accompanying drawing is a diagrammatic View of a four-wiresystem provided with compressor lexpander equipments constructed inaccordance with the invention.

Fig. 2 is a diagrammatic view of one terminal station in the systemshown in Fig. l with the interlock circuit between the compressor andthe expander shown in detail.

Fig. 3 is a diagrammatic view of a terminal station with a modiedinterlock circuit between the compressor and the expander.

Referring to Fig. l of the drawing, a four-wire system is showncomprising a west-to-east channel 2 and an east-to-west channel 3. Thetwo channels 2 and 3 of the four-wire system are connected at a westterminal station 4 by a hybrid coil 5 to a two-wire line comprisingconductors 6 and 1. At an east terminal station 8 of the four-wiresystem the channels 2 and 3 are connected by a hybrid coil 9 to atwo-wire circuit comprising conductors II and I2. Networks I3 and I4 areassociated with the hybrid coils 5 and 9. The west-to-east channel 2comprises conductors I5 and I6 into which are connected a compressor atthe west station 4 and an expander at the east station 8. Similarly, theeast-to-west channel 3, comprising conductors I1 and I8, has connectedtherein a compressor at the east terminal station 8 and an expander atthe west terminal station 4.

The compressor at the west terminal station 4 on the channel 2 comprisesa variolosser I 9 which is controlled by a backward-acting controlcircuit comprising an amplifier 20 and a rectifier 2|. The expander atthe west terminal station 4 comprises a variolosser 22 which iscontrolled by a forward-acting control circuit comprising an ampliiier23 and a rectier 24. Between the variolosser I9 in the channel 2 and thevariolosser 22 in the channel 3 is connected an interlock circuit 25which includes the rectiers 2I and 24. The interlock circuit 25, whichwill be described more in detail when reference is made to Figs. 2 and 3of the drawing, serves to interlock the two variolossers I9 and 22 sothat they may be controlled from either of the channels 2 and 3.

At the east terminal station 3 on the four-wire circuit the compressorconnected to the east-towest channel 3 is provided with a variolosser26. The variolosser 26 is controlled by a backwardacting circuitcomprising an amplifier 2I and a rectifier 28. The expander in thechannel 2 at the east terminal station 8 of the four-wire circuitcomprises a variolosser 29 which is controlled by a forward-actingcircuit comprising an amplier 3@ and a rectiiier 3| The two variolossers29 and 26 are connected by an interlock circuit 32 which includes therectiers 3I and 28. The interlock circuit 32 at the east terminalstation 8, the same as the interlock circuit 25 at the west station 4,serves to insure that operation of each variolosser may be effected byeither channel of the four-wire system.

Referring to Fig. 2 of the drawing, the west terminal station 4 is shownmore in detail. 'Ihe variolosser I9 in the transmitting channel 2 isshown comprising a bridge 33 composed of copper-oxide rectifierelements. If so desired, the bridge may be composed of any othersuitable elements having a non-linear coefficient of resistance. Twoopposite vertices of the bridge 33 are connected across the channel 2adjacent resistance elements 34, 35, 36 and 31 to form a resistance pad.A resistance element 38 is also connected in shunt across the channel 2.The resistance element 38 determines the minimum loss of the variolosserI9. The variolosser I9 is connected to the hybrid coil 5 by means oftransformer 39. An amplifier 49 is preferably associated with thevariolosser. The backwardacting control circuit comprising the amplifier29 and the rectifier 2| is provided for controlling the variolosser I9to compress the transmitted signals. 'Ihe rectifier 2| is preferably inthe form of a bridge composed of copper-oxide rectifier elements. Twovertices of the bridge 2| are connected to the amplifier 20 and theother two vertices of the bridge are connected across the interlockcircuit 25. A condenser 43 and resistance elements 44 and 45 areconnected to the interlock circuit 25 between the bridge 2| and thevariolosser I9 in order to provide a time constant in the control of thevariolosser. This time constant may be Varied, if desired, but ispreferably of the order of forty milliseconds.

'Ihe variolosser 22 in the expander at the West terminal station 4comprises copper-oxide elements 45 shunted by a resistance 4l in oneside of the channel 3 and copper-oxide elements 48 shunted by aresistance element 49 in the other side of the channel 3. Thecopper-oxide elements 4B and 48 are directly connected in the channel 3and vary the gain on the channel 3 directly in accordance with theirresistance. The variolosser 22 is connected to the hybrid coil 5 bymeans of a transformer 59 and an amplifier 5| of any suitable type. Orione side of the copper-oxide elements 46 and 48 of the variolosser 22 aresistance element 52 is connected across the channel 3 and on the otherside of the copper-oxide elements 43 and 48 is connected a transformer53. Control for the copper-oxide elements 46 and 48 is effected from amid-point of the resistance 52' and a mid-point of the secondary windingof the transformer 53. The variolosser 22 is controlled by theforward-acting control circuit comprising the amplifier 23 and therectifier 24. The rectifier 24 is in the form of a copper-oxide bridgesimilar to the bridge 2| in the control circuit for the variolosser I9.Two vertices of the bridge 24 are connected across the interlock circuit25 and the other two vertices of the bridge are connected to the amplier23. A condenser 4| and a resistance element 42 are connected to theinterlock circuit 25 between the bridge 24 and the variolosser 22 inorder to provide a time constant in the operation of the variolosser.This time constant may be changed, if desired, and preferably is of theorder of about forty milliseconds. It should be noted in the circuitarrangement, shown in Fig. 2 of the drawing, that the time constant forthe variolosser I9 and for the variolosser 22 are controlled by thecondensers 4| and 43 and the resistance elements associated with thecondensers. This condition is apparent because the two condensers 4| and43 are connected in parallel across the interlock circuit 25.

In the system, as shown in Fig. 2 of the drawing, incoming speechcurrents on the two-wire line comprising conductors 6 and 'I will betransmitted over the channel 2 to the distant terminal station 8 of thefour-wire system. The variolosser I9 under control of the amplifier 20and the rectifier 2| will compress the transmitted signals and at thesame time the potential from the rectifier 2| will operate thevariolosser 22 to increase the gain on the channel 3. The increase inthe gain on the channel 3 will only take place when the speech currentsare high on the channel 2 and will not cause any bad effect unless thetalker should be at a distance from the terminal station of thefour-Wire system. When speech or noise currents are received over thechannel 3, the variolosser 22 is operated by the amplifier 23 and therectifier 24 to expand the volume of the received signals. At the sametime, the variolosser I9 in the channel 2 is operated by the potentialfrom the bridge 24 to lower the gain on the channel 2. The lowering ofthe gain on the channel 2 will insure against any increased interferenceby singing or echoes. The two bridge circuits 2| and 24 are so poledwith respect to each other that simultaneous control of the two bridgesfrom the two channels 2 and 3 cannot take place at the same time. Thebridge which has the highest potential impressed thereon will assumecontrol and bias the other bridge against operation.

Referring to Fig. 3 of the drawing, a modification of the interlockcircuit, shown in Fig. 2, is illustrated. The circuits shown in Fig. 3are similar to the circuits shown in Fig. 2 with the exception of theinterlock circuit and like parts have been indicated by similarreference characters. In the interlock circuit 25 shown in Fig. 3 of thedrawing, unilateral devices 55 are provided for insuring that thevariolosser 22 in the channel 3 will not be operated from the channel 2.The variolosser I9 in the channel 2 may be operated from the channel 3simultaneously with the variolosser 22. The unilateral elements 55 arepreferably copper-oxide discs which serve to effect flow of current inthe interlock circuit in one direction only. Signal currents or noisecurrents on the channel 3 operate the amplifier 23 and the bridge 24 toeffect operation not only of the variolosser 22 to effect expansion butalso to effect operation of the variolosser I9 in the channel 2 toeffect compression in the channel 2. The simultaneous operation of thetwo variolossers I9 and 22 insure against any increased trouble fromsinging or echoes. A biasing battery 60 may be added to the interlockcircuit 25 which is so poled that the expander rectifier voltage mustexceed the bias to effect any operation of the variolosser I9 in thechannel 2. The addition of the biasing battery 60 may be advantageous ifnoise in the channel 3 becomes so high that the expander controlfrequently reduces, the compressor gain during transmission of weakspeech currents. In the circuit shown in Fig. 3 of the drawing, thevariolossers I9 and 22 may be operated with different time constants. Itis possible to hold the loss caused by the variolosser I9 on thetransmitting channel 2 for a period of time after the expandervariolosser 22 has returned to normal condition. The holding of the lossin the transmitting channel for an extended period of time is desirablein the case of delayed echoes.

Although no radio apparatus is shown on the drawing, it is to beunderstood that, if so desired, a portion of the four-wire circuit mayinclude a radiol link. The circuit arrangement shown in Fig. 2 of thedrawing will prevent trouble from echoes and singing due to the variablegain compressors and expanders under most operating conditions. Undersome noise conditions, it may be desirable to employ the circuitarrangement 1 shown in Fig. 3 of the drawing if the circuit hassufficient singing margin.

Modications in the circuits and in the arrangement and location of partsmay be made within the spirit and scope of the invention. Suchmodications are intended `to be covered by the appended claims.

What is claimed is:

1. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section, twovariolossers respectively located in the transmitting and receivingchannels to eiect compression and expansion of signals, an interlockcircuit between said variolossers, control means connecting saidinterlock circuit to said channels for compressing transmitted signalsand expanding received signals, and means for connecting said controlmeans to said interlocked circuit in a manner to prevent simultaneouscontrol of the two variolossers from the two channels.

2. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section, twovariolossers respectively located in the transmit- .ting and receivingchannels to effect compression and expansion of signals, an interlockcircuit between said variolossers, two bridge circuits formed ofelements having non-linear coeicients of resistance and each having twoopposite vertices connected across said interlock circuit, meanscontrolled by the signals on the transmitting channel and connected totwo vertices of one bridge for controlling the variolosser in thetransmitting channel to effect a compression operation and meanscontrolled from the receiving channel and connected to two oppositevertices of the other bridge for controlling the two variolossers toe'ect expansion on the receiving channel and attenuation on thetransmitting channel, said bridge circuits being poled so that only onebridge circuit can effect a` control operation at one time.

3. In a signal transmission system having a four-wire section withtransmitting and receiving `channels connected to a two-wire section,two

variolossers respectively located in the transmitting and receivingchannels to effect compression and expansion of signals, an interlockcircuit connected between said variolossers, two bridge cirrcuitscomposed of copper-oxide elements connected across said interlockcircuit, said bridge circuits being similarly poled with respect to theinterlock circuit, and means comprising a backward-acting controlcircuit for controlling one of said bridges from the transmittingchannel to govern the variolosser in the transmitting channel and effectcompression on the transmitting channel, a forward-acting controlcircuit for controllingr the other bridge from the receiving -channel togovern both of said variolossers and effect a loss in the transmittingchannel and expansion in the receiving channel, the attenuation on saidchannels being governed by the variolossers under control of the controlcircuits to prevent singing action.

4. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section by ahybrid coil, a variolosser in the transmitting channel to effect signalcompression, a variolosser in the receiving channel to effect signalexpansion, an interlock circuit between said variolossers, meansconnected to the interlock circuit for controlling said variolossersoppositely from the transmitting channel, means connected to theinterlock circuit for controlling said variolossers oppositely from thereceiving channel, and means for connecting said last two means to theinterlock circuit to insure operation of the variolossers only from onechannel at the same time.

5. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section, avariolosser in the transmitting channel to e'ect signal compression, avariolosser in the receiving channel to eiect signal expansion, aninterlock circuit between said variolossers, a condenser resistancearrangement inserted in said interlock circuit for providing a timeconstant in operating the variolossers, means connected to saidinterlock circuit for operating said variolossers from the transmittingchannel, means connected tothe interlocked circuit for operating saidvariolossers from the receiving channel and means for connecting saidlast two means to the interlocked circuit to insure operation of thevariolossers under control of the signals in one channel at a time.

6. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section by ahybrid coil, a variolosser in the transmitting channel, a variolosser inthe receiving channel, an interlock circuit between said variolossers,two bridge circuits formed of elements having nonlinear coefcients ofresistance and each having two opposite vertices connected across saidinterlock circuit, means controlled by the signals on the transmittingchannel and connected to two vertices of one of said bridges forcontrolling the variolosser in the transmitting channel to effectcompression of the signals and for controlling the variolosser in thereceiving channel in an opposite manner, means controlled from thereceiving channel and connected to the other bridge for controlling thetwo variolossers to expand the signals on the receiving channel and toadd loss on the transmitting channel, said bridge circuits being poledso that only one bridge can control the variolossers at one time.

7. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-wire section, avariolosser in the transmitting channel to effect signal compression, avariolosser in the receiving channel to effect signal expansion, aninterlock circuit between said variolossers, unilateral transmittingmeans inserted in said interlock circuit, means connected to theinterlock circuit on one side of said unilateral transmitting means forcontrolling the variolosser in the transmitting channel from thetransmitting channel, and means connected to the interlock circuit onthe other side of the unilateral transmitting means for controlling thevariolossers in each of said channels from the receiving channel.

8. In a signal transmission system having a four-Wire section withtransmitting and receiving channels connected to a two-wire section, twovariolossers respectively located in the transmitting and receivingchannels to effect compression and expansion of signals, an interlockcircuit between said variolossers, transmitting control means connectedbetween the transmitting channel and the interlock circuit for governingthe variolosser in the transmitting channel to effect a compressionoperation, receiving control means connected between the receivingchannel and the interlock circuit for governing the variolosser in thereceiving channel to eiect an expanding operation and for governing thevariolosser in the transmitting channel to effect attenuation, and meansin the interlock circuit for preventing operation of the variolosser inthe receiving channel by the transmitting control means.

9. In a signal transmission system having a four-wire section withtransmitting and receiving channels connected to a two-Wire section, twovariolossers respectively located in the transmitting and receivingchannels to effect compression and expansion of signals, an interlockcircuit between said variolossers, two bridge circuits formed ofelements having non-linear coeicients of resistance and each having twooppo site vertices connected across said interlock circuit, unilateraltransmitting means comprising copper-oxide rectier elements inserted insaid interlock circuit between the connections of said bridges to theinterlock circuit, means controlled by the signals on the transmittingchannel and connected to the bridge adjacent to the variolosser in thetransmitting channel for controlling the variolosser in the transmittingchannel to eiect a compression operation, said unilateral meanspreventing operation of the variolosser in the receiving channel by thesignals on the transmitting channel, and means controlled from thereceiving channel and connected to the other bridge for controlling thetwo variolossers to effect expansion in the receiving channel and a lossin the transmitting channel.

10. In a signal transmission system having a four-wire section Withtransmitting and receiving channels connected to a two-Wire section, avariolosser in the transmitting channel to effect signal compression, avariolosser in the receiving channel to eiTect signal expansion, aninterlock circuit between said variolossers, unilateral transmittingmeans inserted in said interlock circuit, rectifying means connectedbetween the transmitting channel and the interlock circuit on one sideof said unilateral transmitting means for controlling the variolosser inthe transmitting channel, receiving rectifying means connected betweenthe receiving channel and the interlock circuit on the other side ofsaid unilateral transmitting means for controlling both variolossers toexpand the signals on the receiving channel, and to add loss to thetransmitting channel, and a biasing potential inserted in said interlockcircuit for preventing operation of the variolosser in the transmittingchannel from the receiving channel until the receiving rectifying meanssupplies potential to the interlock circuit above the potential of thebias.

ANDREW C. NORWINE.

